Asked by: Jennifer Armstrong, Maidstone
Despite the marketing, colour change has absolutely nothing to do with mood and everything to do with acidity (pH). The lipsticks contain dyes that act a bit like litmus paper, the acidity indicator beloved of school chemistry teachers.
Inside the lipstick, the dyes are colourless, weak acids. But lips have a higher pH than the lipstick, which triggers a chemical reaction that converts the acids into a strongly coloured compound.
The exact lipstick colour will depend on the skin’s pH, which is in turn affected by physiological factors such as physical activity and hormone fluctuations. Our underlying natural lip colour also subtly affects the final lipstick shade.
Subscribe to BBC Science Focus Magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.